1. Field of the Invention
This invention relates generally to an apparatus for and method of measuring an unknown quantity, and more particularly to an apparatus for and method of determining when the value of an unknown quantity is within a desired range of a reference quantity.
2. Prior Art
It is often desirable to determine when the value of an unknown quantity is within a desired range of a reference quantity. For example, many processes employ chemical agents which must be conveyed at prescribed flow rates from one station to another, and it is often necessary to maintain those flow rates within a prescribed range. In such a case, the reference quantity is the desired flow rate of a particular chemical agent and the desired range is either a percentage of that flow rate or an arbitrary amount, such as a prescribed number of units. Accordingly, in this example, the value of the reference quantity, and the desired range with respect to that reference quantity are known.
In some instances, however, the value of the reference quantity is not known. For example, when it is desired to compare some characteristic of one material, such as a standard or a reference, with the same characteristic of another material to determine whether the quantitized values of such characteristics are within a prescribed range of one another, the quantitized value of the examined characteristic of the reference material is not known. This is the case when, for example, light reflectance from one material is to be compared with the light reflectance from another material.
In the past, this function was often performed mentally by an operator observing a meter connected to an instrument for measuring the unknown quantity and comparing the meter reading with the value of the reference quantity. If the reference quantity is unknown, such as in the second example described above, another meter connected to a second instrument for measuring the reference quantity is observed by an operator and that meter reading is compared with the measured value of the unknown quantity.
It can be appreciated that this method of determining when an unknown quantity is within a desired range of a reference quantity is subject to errors. Since the comparison requires visual inspection by an operator, the unknown quantity may vary outside the desired range without detection. Furthermore, errors can occur in any visual interpretation and analysis of data in performing such a comparison. Visual interpretation and analysis of data are also time consuming functions, not only increasing labor costs, but delaying the acquisition of the results of the comparison.
In order to overcome the problems associated with an operator observing instrument readings and visually interpreting and analyzing such readings, attempts have been made to automatically sense the reference quantity and the unknown quantity and to automatically compare one with the other to provide an indication, such as an alarm, whenever the unknown quantity varies outside a prescribed range with respect to the reference quantity. One such attempt involves storing a voltage corresponding to the value of the sensed reference quantity, and comparing the stored value with a voltage corresponding to the sample unknown quantity. The comparison is performed by reducing both voltages by a prescribed amount which corresponds to the desired range, and comparing the full value of each voltage with the reduced value of each voltage in two comparators.
This analog technique for determining when an unknown quantity is within a desired range of a reference quantity requires constant adjustment due to drift of the components and calibration of the range settings. If it is desired to maintain a range having arbitrary limits, such as plus and minus a prescribed number of units (percent of full scale, rather than percent of readings), the operator of the above described apparatus would have to calculate those arbitrary limits as percentages of the reference. However, since the value of the reference quantity is unknown, such calculation can not be performed without additional instruments to measure the reference quantity. Such human involvement in reading meters and calculating the range limits would, of course, present the same problems which are mentioned above.
Other analog techniques are also empolyed for performing one or more of the above described functions. However, these techiques also suffer from the same problems discussed above. Usually, such techniques are only capable of providing a range which is a percentage of full scale (arbitrary limits) or a percentage of reading. If it is desired to determine when the unknown quantity is within a range which is a percentage of reading and the circuit can only be set to provide a range as a percentage of full scale, calculations are required to convert from one to the other. Since the reference quantity may change from time to time, it would be necessary to recalculate the range for each new reference quantity.
Another problem occurs in the storage of the reference quantity. If a voltage corresponding to the value of the reference quantity is stored on a capacitor, for example, that voltage will begin to decay as it is being compared with the voltage corresponding to the sampled quantity. Accordingly, if the reference quantity remains fixed over a relatively long time period, the stored voltage corresponding thereto must be periodically replenished.
Accordingly, it can be appreciated that a need exists for an apparatus which is capable of storing a quantity corresponding to a reference quantity for an indefinite period of time and comparing that stored quantity with a sampled quantity to determine whether the sampled quantity is within a desired range of the reference. Furthermore, a need exists for such an apparatus which does not require visual interpretation and analysis of data in order to perform that comparison. It is also desirable that such an apparatus be capable of establishing a range which is either a percentage of full scale or a percentage of reading.